A typical example of such instruments includes medical optical instruments such as surgical microscopes, slit-lamps, retinal cameras and other instruments. These medical optical instruments comprise an optical illuminating system for illuminating an object to be observed and an optical observing system for observing the object.
Such optical instruments are classified into an independent type in which the illuminating field of the optical illuminating system can be changed completely independently of the magnification of the optical observing system and an interlocking type in which both the optical systems can be interlocked at all times.
The independent type optical instruments are troublesome in operation since the illuminating system should be independently adjusted to change its illuminating field for each change of magnification in the optical observing system.
On the other hand, the interlocking type optical illuminating and observing instruments are not troublesome in operation. However, they can always be used only in combinations of lower observing magnification--wider illuminating field and higher observing magnification--narrower illuminating field. Therefore, the interlocking type optical instruments cannot meet various requirements where a part of the observing field or a part of an object to be observed should be more brightly illuminated although the observing magnification may not be very increased with the illuminating field being decreased or where the effective exit pupil of the illuminating system is to be located nearer the optical axis for observation as in the operation on the cataract by the use of a surgical microscope.
It is therefore an object of the present invention to overcome the aforementioned disadvantages of the optical instruments according to the prior art and to provide an optical illuminating and observing apparatus having the advantages in both the independent and interlocking type optical instruments.